1. Field of the Invention
The present invention relates to a switching power supply and, more particularly, to a driving method of a DC/DC converter.
2. Description of the Related Art
To generate a voltage higher than an input voltage, a step-up switching power supply is widely used in various electronic devices. Such a step-up switching power supply has a switching element and an inductor or transformer. By turning on/off the switching element in a time sharing manner, back electromotive force is generated in the inductor or transformer to boost the input voltage, and the boosted voltage is output.
In an insulating-type DC/DC converter, when a switching transistor is turned off, current flows through the primary side of a transformer, and energy is stored in the transformer. When the switching transistor is turned off, the energy stored in the transformer on the secondary side of the transformer is transferred as charge current via a rectifier diode to an output capacitor, so that an output voltage increases. When the energy stored in the transformer is transferred to the output capacitor, the current flowing through the rectifier diode becomes zero.
As one of isolated DC/DC converters, a self-exciting DC/DC converter is known which monitors the state on the primary side or secondary side of a transformer without using an oscillator, and controls turn-on/off of a switching transistor in accordance with the state (refer to, for example, Japanese Patent Application Laid-Open Nos. 2004-201474 and 2005-73483).
As a method of controlling a self-exciting DC/DC converter, a control method including monitoring the current flowing through the primary and secondary sides of the transformer, turning on a switching transistor at a timing when current flowing through the secondary side becomes zero, and turning off the switching transistor at a timing when current on the primary side reaches a predetermined level will be considered.
In the case of controlling the switching transistor by the control method described above, in a state where the switching transistor is off, energy stored in the transformer is transferred to an output capacitor. In this case, if the switching transistor is switched to the on state immediately when the current flowing through the secondary side drops to nearly zero, then the switching transistor is turned on again in a state where energy still remains in the transformer due to the influence of reverse recovery time of a rectifier diode and the like. It deteriorates the efficiency of the DC/DC converter.